1. Field of the Invention
The present invention relates to the field of combustion engine diagnostics. More specifically, the present invention is directed to methods and apparatus by which the performance of the combustion process in individual cylinders of a multi-cylinder engine can be monitored and low power level conditions detected.
2. Description of Related Art
The cylinder pressure resonance phenomenon is well known and understood. Furthermore, as reflected by U.S. Pat. Nos. 4,895,121 (McCoy et al.), 5,119,783 (Komurasaki) and 5,144,929 (Hosoya et al.), this phenomenon is used in methods and apparatus for controlling engine operation for suppressing the occurrence of autoignition, i.e., knocking. In accordance with these disclosures, vibration sensors are used to monitor engine cylinder resonant frequencies produced during operation of a multi-cylinder engine, and the magnitude of the resonance frequency peaks are evaluated as a knocking indicator. The frequency range in which the resonant frequency occurs is not taken into consideration and the only abnormal combustion condition detected is knocking, except for Komurasaki which also detects misfiring.
Additionally, in "Ein nuer Klopdetektor fur Ottomoteren, Ein neues, empfindlicheres Verfahren zur Detektion yon Klopfen bei Ottomotoren" [A New Knock Detector for Spark-Ignition Engines, a new more sensitive method of detecting knock in spark-ignition engines], N. Harle and J. F. Bohme, Automobil-lndustrie, January, 1990, describes a more accurate manner of determining the existence of engine knocking using a microprocessor to perform complex vibration signal processing tasks including signal transformation, weighting, etc. By the described techniques, it is indicated that greater sensitivity can be obtained for determining that a detected level of resonance energy has exceeded a predetermined threshold indicative of knocking while minimizing the probability of a false indication of knocking being issued.
Tests for the identification of low power cylinders in multi-cylinder engines which are not dependent on the detection of knocking are known. However, these tests can only be performed during engine idling condition through the use of cylinder cut-out techniques. Thus, such tests are not suitable for an on-board vehicle diagnostic system, and cannot be used to perform diagnostic tests under on-the-road driving conditions.